This invention relates to a method and apparatus for forming large articles having curved surfaces from plastic sheet material.
Vacuum forming methods for forming large articles having curved surfaces from sheets of vinyl or plastic material are well known in the art. These processes are commonly used to form large automotive body members, such as bumper facia, and may also be used to form tank halves and similar articles. According to a known process, a plastic sheet is mounted in a clamping frame, heated, and then formed by forcing a die into the sheet and then sucking the sheet against the die by use of vacuum. The product is then cooled, the die extracted, and the finished product removed from the clamping frame. This and similar processes have been widely used to manufacture such articles at a relatively low cost.
However, prior art processes, depending upon the shape and complexity of the article being formed, require larger sheets than would otherwise be necessary to accommodate stretching as the mold enters the sheet. Accordingly, the thickness of the product cannot be easily controlled, and “chills” and other defects in the product are common. Furthermore, dies must be carefully designed when complex shapes are formed to avoid die locks in which it is impossible to remove the die from the finished part.